[clang] serialize SUBMODULE_TOPHEADER relative to BaseDirectory

This diff changes the serialization of the `SUBMODULE_TOPHEADER`
entry in module files to be serialized relative to the module's
`BaseDirectory`. This matches the behavior of the
`SUBMODULE_HEADER` entry and will allow for the module to be
relocatable across machines.

The path is restored relative to the module's `BaseDirectory` on
deserialization.

Reviewed By: urnathan

Differential Revision: https://reviews.llvm.org/D124938